Macromolecular engineering of complex molecular architectures through the introduction of controlled branching has become an increasingly important theme in polymer science. Accordingly, the synthesis of branched molecular architectures, such as hyperbranched, star, dendritic, and combburst macromolecules, is the goal of many research groups. This interest is driven by the unique mechanical and rheological properties of these nanoscopically tailored materials. Both dendrimers and hyperbranched macromolecules are prepared from AB.sub.2 monomers although the different synthetic strategies lead to significantly different structures. Dendrimers are prepared by a stepwise series of reactions using either the divergent growth approach, which starts from a multifunctional core molecule and proceeds radially outward, or the convergent growth approach in which the synthesis starts at the periphery and well-defined dendrons are prepared followed by final coupling to a multifunctional core.
The repetitive nature of dendrimer synthesis, while tedious, does lead to an essentially perfectly branched structure. In contrast, hyperbranched macromolecules are prepared in a single-step polymerization process from AB.sub.x monomers, which greatly facilitates their availability but leads to polymers with irregular branching and broad molecular weight distributions are obtained.
Commercially available hyperbranched polyesters, derived from 2,2bis(hydroxymethyl) propionic acid (bis-MPA), have recently been introduced and are attractive for a number of technological applications. Other of hyperbranched polymers include those disclosed by Hult et al., U.S. Pat. No. 5,418,301 which teaches dendritic macromolecules of polyester, the initiator polymer having reactive hydroxyl groups. Sorensen et al., U.S. Pat. No. 5,663,247 also teaches hyperbranched macromolecules of the polyester type having at least one branching chain extender which has at least three reactive sites of which at least one is a hydroxyl or hydroxyl alkyl and at least one is a carboxyl or terminal epoxide group. The nucleus is an epoxide compound having at least one reactive epoxide group.
However, one of the drawbacks of all of these highly branched macromolecules is the inability of the globular, densely packed structures to entangle and to provide suitable mechanical properties. The resulting brittle materials limits the range of applications for which dendrimers or hyperbranched macromolecules are suitable.
As a result, there is a need for hyperbranched polymers which provide the desired mechanical and physical properties.